1. Field of the Invention
This invention relates to an imaging equipment exploiting ultrasonic radiation, and more particularly to an acoustic microscope.
2. Description of the Prior Art
In recent years, it has become possible to generate and detect acoustic waves at ultra high frequencies reaching 1 GHz and therefore to realize an acoustic wavelength of approximatly 1 .mu.m in the water. As a result, it has become possible to fabricate an acoustic imaging equipment of high resolution. The equipment forms a focused ultrasonic beam with a concave lens, thereby to realize the resolution as high as 1 .mu.m.
A specimen is inserted in the beam, and an ultrasonic wave reflected by the specimen is detected, whereby the elastic properties of a very small area of the specimen are elucidated. Alternatively, while a specimen is being mechanically scanned in two dimensions, the intensity of the resulting signal is displayed as a brightness signal on a cathode-ray tube, whereby a very small area of the specimen can be observed on an enlarged scale.
First, a prior-art construction of such ultrasonic microscopic imaging equipment will be described, and a problem involved therein will be pointed out.
FIG. 1 is a view which shows the schematic construction of a prior art of a transducer system for obtaining a reflected signal from a specimen (as disclosed in, for example, U.S. Pat. No. 4,028,933). An ultrasonic propagating medium 20 (cylindrical crystal of, for example, sapphire or silica glass) has one end face 21 which is an optically polished plane, and the other end face which is formed with a concave semispherical hole 30. An RF pulse ultrasonic wave which is a plane wave is radiated into the crystal 20 by an RF pulse signal which is impressed on a piezoelectric film 10 deposited on the end face 21. The plane ultrasonic wave is focused on a specimen 50 located on a predetermined focal point, by a positive acoustic lens which is formed by the interface between the semispherical hole 30 and a medium 40 (in general, water).
The ultrasonic wave reflected and scattered by the specimen 50 is collected and converted into a plane wave by means of the same lens. The plane wave is propagated through the interior of the crystal 20, and is finally converted into an RF electric signal by the piezoelectric film 10. The RF electric signal is detected by a diode into a video signal, which is used as the input signal of the cathode-ray tube as stated above.
FIG. 2(a) shows detected signals in the video region at the time when, in such prior-art construction, an RF pulse signal having a certain repetition rate t.sub.R was impressed. Here, the axis of abscissas is a time axis and the axis of ordinates represents the intensity of the signal. Letter A designates the applied RF pulse, letter B a reflected signal from the lens boundary, and letter C a reflected signal from the specimen.
In order to discriminate the desired reflected signal C from the reflected signal B, the prior-art imaging equipment adopts a construction in which the duration t.sub.d of the impressed pulse is shortened to the utmost so as to prevent the signals C and B from overlapping each other, whereby only the signal C is taken out by a timing gate as shown in FIG. 2(c).
The resolutions of such equipment include an axial resolution .DELTA..zeta. in the direction of propagation of the ultrasonic wave, and a lateral resolution .DELTA..gamma. within a plane perpendicular to the propagating direction of the ultrasonic wave. Both are determined by the wavelength .lambda. of the ultrasonic wave and the F number representative of the brightness of the lens used, and are given by: EQU .DELTA..gamma.=.lambda..multidot.F (1) EQU .DELTA..zeta.=2.lambda..multidot.F.sup.2 ( 2)
Since the F number of the lens which can be fabricated is approximately 0.7, .DELTA..gamma..about.1 .mu.m and .DELTA..zeta..about.1.5 .mu.m hold in the water (1,500 m/s) when the ultrasonic wave used is at 1 GHz.
However, an IC or LSI which is the most important object to-be-imaged of the ultrasonic microscope requires a better axial resolution. This is because, in the IC, a layered pattern is often finer than a planar pattern as is well known. In actuality, a typical IC has a multilayered structure consisting of layers 1 .mu.m-3 .mu.m thick. With the axial resolution of 2 .mu.m in the water as above stated, it is utterly impossible that the layers are nondestructively observed independently of one another with the position of a focal point set inwardly of the surface of the IC. The reason is that, since the acoustic velocity is higher in a metal such as silicon and aluminum which is the material of the IC than in the water, the axial resolution is merely 4-10 .mu.m even when the ultrasonic wave at 1 GHz is used.